Indole-3-ethanol oxidase: kinetics, inhibition, and regulation by auxins

We report the further characterization of indole-3-ethanol oxidase from cucumber seedlings. The effects of various inhibitors suggest that the enzyme may be a flavoprotein with a metal ion and sulfhydryl groups required for full activity. Indole-3-acetaldehyde, a product of the reaction, inhibits th...

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Bibliographic Details
Published in:Plant physiology (Bethesda) 1973-04, Vol.51 (4), p.739-743
Main Authors: Percival, F.W, Purves, W.K, Vickery, L.E
Format: Article
Language:English
Subjects:
Auxins
Biosynthesis
Cucumbers
Enzymes
Indoleacetic acids
Insulin antibodies
Kinetics
Oxidases
Physiological regulation
Seedlings
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Summary:We report the further characterization of indole-3-ethanol oxidase from cucumber seedlings. The effects of various inhibitors suggest that the enzyme may be a flavoprotein with a metal ion and sulfhydryl groups required for full activity. Indole-3-acetaldehyde, a product of the reaction, inhibits the enzyme. This inhibition is overcome by O2 but not by indole-3-ethanol, indicating that the kinetic mechanism of the enzyme is a pingpong Bi-Bi. The enzyme undergoes cooperative interactions with indoleethanol, yielding Hill coefficients as high as 2.96. Gibberellins are without effect on the enzyme, but it is inhibited by several acidic indoles possessing growth-promoting activtiy and by two synthetic auxins, 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid. Increasing concentrations of indoleacetic acid (IAA) brought about a slight reduction in the indoleethanol concentration producing half-maximal velocity. Increasing levels of indoleethanol decreased the concentration of IAA required for half-maximal inhibition. At low concentrations of indoleethanol, low levels of IAA activated rather than inhibited. The effect of IAA was not overcome at higher levels of indoleethanol. These results may be interpreted as showing that IAA is a noncompetitive inhibitor which binds to that conformation of the enzyme which also binds indoleethanol. The significance of these interactions for the regulation of IAA biosynthesis is discussed.
ISSN:0032-0889
1532-2548
DOI:10.1104/pp.51.4.739